Since after being cured, silicone compositions form silicone gels that are characterized by excellent stress-buffering properties, electrical properties, resistance to heat, and weather-proof properties, they find application for sealing and filling various electrical and electronic components (see Japanese Patent Publications [Kokai] No. 59-204259, No. 61-48945, and No. 62-104145). For example, Japanese Patent Publication [Kokai] No. 48-17847 describes a composition comprising a branch-chained vinyl-containing organopolysiloxane having a viscosity of 0.01 to 10 Pa.s at 25.degree. C., a linear diorganopolysiloxane having silicon-bonded hydrogen atoms, and a hydrosilylation catalyst; Japanese Patent Publication [Kokai] No. 58-7452 discloses a silicone gel composition comprising a branched vinyl-containing organopolysiloxane and a hydrosilylation catalyst; and Japanese Patent Publication (Kokoku) No.3-19269 discloses a silicone gel composition comprising a branched vinyl-containing organopolysiloxane having a viscosity of 0.02 to 10 Pa.s at 25.degree. C., a linear diorganopolysiloxane with both molecular-chain terminals sealed with vinyl groups, a linear diorganopolysiloxane having silicon-bonded hydrogen atoms only at both molecular-chain terminals, and a hydrosilylation catalyst.
However, when the aforementioned silicone gel compositions are used for sealing or filling electrical and electronic components of complicated structure, heat cycles during the operation of these components may cause formation of air bubbles and cracks in the silicone gel used for sealing or filling, whereby reliability of the aforementioned electrical and electronic components is decreased. It can be assumed that air bubbles and cracks occur in the silicone gel that fills aforementioned electrical and electronic components because such components have very narrow spaces between electrodes, electrical elements, electrical elements and the package, etc., as well as because silicone gel material itself is subject to expansion and shrinkage.
Furthermore, in order to make a judgment about the quality of silicone gel used for sealing and filling electrical and electronic components, it is required that this silicone gel be transparent or semitransparent. However, the transparent or semitransparent silicone gel has poor combustion-resistance properties and therefore it is not applicable for sealing or filling those electrical or electronic components which are designed for long-term use under elevated temperatures.